1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly, to a plasma display panel that is capable of improving a contrast of a plasma display panel.
2. Description of the Background Art
In general, a plasma display panel is a display device which excites a fluorescent layer by using an ultraviolet ray of plasma and displays an image by using a visible light generated from the fluorescent layer.
In a conventional plasma display panel, a contrast is degraded due to a near infrared ray or a visible light generated from a discharge gas and an external visible light reflected after being irradiated from outside. Thus, in the conventional art, in order to improve the contrast, a liquid crystal filter is adopted to the plasma display panel.
In case of adopting the liquid crystal filter to the plasma display panel, a fabrication process is complicated to degrade an yield and a fabrication cost is increased.
The structure of the conventional plasma display panel adopting the liquid crystal filter will now be described with reference to FIG. 1.
FIG. 1 is a sectional view showing the structure of the conventional plasma display panel.
As shown in FIG. 1, the conventional plasma display panel includes: a lower insulation film 9 formed on a lower glass substrate 10; an address electrode 11 formed on a certain portion of the lower insulation film 9; a lower dielectric layer 8 formed on the address electrode 11 and on the lower insulation film 9; a barrier rib 7 defined at a certain portion on the lower dielectric layer8 to divide discharge cells; a black matrix layer 12 formed on the barrier rib 7; a fluorescent layer 13 formed with a certain thickness on the side of the black matrix 12 and the barrier rib 7 and on the lower dielectric layer 8, and emitting each red, green and blue visible light upon receiving an ultraviolet ray; an upper glass substrate 2; a sustain electrode 3 formed on a certain portion of the upper glass substrate 2 so as to intersect with the address electrode 11 with a certain space vertically therebetween; a bus electrode 5 formed on a certain portion of the sustain electrode 3; an upper dielectric layer 4 formed on the bus electrode 5, the sustain electrode 3 and the upper glass substrate 2; a passivation layer 6 formed on the upper dielectric layer 4 in order to protect the upper dielectric layer 4; and a liquid crystal filter 1 formed on the upper glass substrate 2 and cutting off or transmitting the visible light emitted from the plasma display panel according to whether a voltage is supplied thereto.
The operation of the conventional plasma display panel will now be described.
First, an SLS (Soda-lime Silicate (SLS)) glass substrate is used as the upper glass substrate 2 and the lower glass substrate 10 of the conventional plasma display panel.
The lower insulation film 9 is positioned on the lower glass substrate 10, the SLS glass substrate, and the address electrode 11 is positioned at a certain portion on the lower insulation film 9.
The lower dielectric layer 8 positioned on the address electrode 11 and the lower insulation film 9 interrupts a visible light emitted in the direction of the lower glass substrate 10. In order to increase a luminous efficacy, a dielectric layerwith a high reflectance is used as the lower dielectric layer8.
The fluorescent layer 13 consists of red, green and blue fluorescent materials, and emits a visible light of a specific wavelength according to an intensity of an ultraviolet ray owing to a plasma generated in the region between the barrier ribs 7.
The sustain electrode 3 is positioned at a lower surface of the upper glass substrate 2, the SLS glass substrate, which intersects the address electrode 11 in a vertical direction. A bus electrode 5 is positioned at a certain portion on the sustain electrode 3, and a dielectric layer4 with an excellent light transmittance is positioned on the dielectric layer4.
The passivation film 6 is positioned on the dielectric layer 4 to prevent a damage of the dielectric layer4 owing to occurrence of plasma.
Thereafter, when a voltage required for a preliminary discharge is applied to the sustain electrode 3, there occurs a voltage difference between the bus electrode 5 and the address electrode 11 which intersects the bus electrode 5 in the vertical direction.
Owing to the voltage difference, the gas positioned at the region between the barrier ribs 7 turns to a plasma state, and a visible light of a specific wavelength is emitted from the fluorescent film 13 due to an ultraviolet ray generated from the plasma.
The preliminary discharge refers to a process of generating an electric charge on the surface of the dielectric (the dielectric layers 4 and 8) in order to increase a driving speed of the plasma display panel. In this case, however, when the electric charge is generated on the surface of the dielectric, a basic discharge is being made even in a state that each discharge cell is in an OFF state, causing a problem that a small amount of (dim) light is generated from each discharge cell.
For example, as for the plasma display panel, the discharge gas in the pixel region defined by the barrier rib 7 becomes plasma due to a potential difference between the address electrode and the bus electrode 5, the fluorescent film 13 is excited by the ultraviolet ray of the plasma to generate the visible light, and an image is displayed by using the visible light.
That is, a desired color is displayed by exciting the fluorescent film 13 by using the ultraviolet ray generated by Xe gas among the discharge gases consisting of He gas, Xe gas and Ne gas injected into the discharge space divided by the barrier ribs 7.
At this time, the plasma display panel should have a high contrast. The contrast herewith refers to a ratio of the brightest portion of a screen to the darkest portion.
For example, the greater the difference between the maximum brightness and the minimum brightness of the display panel is, the more the display characteristics (i.e., the contrast) of the screen is improved.
The maximum brightness can be increased by improving a constitution of phosphor or a fabrication technique, or can be also increased by improving a driving method.
Meanwhile, as the method of increasing the maximum brightness has reached its limit, a method of reducing the minimum brightness is sought. In this respect, however, in the conventional art, the preliminary discharging is performed to increase the driving speed of the plasma display panel. It is difficult to lower the minimum brightness.
Namely, when the electric charge is generated on the surface of the dielectric to increase the driving speed of the plasma display panel, the basic discharging is being made even in the state that each discharge cell is in the OFF state, emitting a small amount of light from each discharge cell.
Thus, in the conventional art, in order to lower the minimum brightness of the plasma display panel, the liquid crystal filter 1 is formed on the upper glass substrate2.
That is, in the preliminary discharge of the plasma display panel, the visible light (the small amount of light) emitted from the plasma display panel is interrupted by applying a voltage to the liquid crystal filter 1, thereby reducing the minimum brightness.
Meanwhile, in case of displaying an image through the plasma display panel on a screen, a voltage applied to the liquid crystal filter 1 is interrupted to transmit the visible light emitted from the plasma display panel to display an image.
The liquid crystal filter 1 includes: a lower electrode (not shown) positioned on the upper glass substrate 2; a liquid crystal (not shown) positioned on the lower electrode and determining a light transmittance upon receiving a voltage; and an upper electrode (not shown) positioned on the liquid crystal.
The liquid crystal filter 1, changing the light transmittance depending on the difference between the voltages applied to the lower electrode and the upper electrode, has a simple filter type by adopting the method used for the liquid crystal display (LCD).
However, in case that the image is displayed through the liquid crystal filter 1 formed on the upper glass substrate, the transmittance of the visible light is reduced as the visible light transmits the liquid crystal, resulting in reduction in the overall luminous efficacy.
In addition, in case that the liquid crystal filer 1 is formed on the upper glass substrate 2, the thickness and weight of the plasma display panel are increased. Moreover, use of the high-priced liquid crystal increases a fabrication cost of the plasma display panel.
Meanwhile, other plasma display panels and their fabrication methods are disclosed in detail in the U.S. Pat. No. 5,838,106 registered on Nov. 17, 1998, a U.S. Pat. No. 6,242,859 registered on Jun. 5, 2001 and the U.S. Pat. No. 6,344,080 registered on Feb. 5, 2002.
As stated above, in the conventional art, since the liquid crystal filter is formed on the upper glass substrate of the plasma display panel in order to improve the contrast by lowering the minimum brightness of the panel, the light transmittance is reduced, which causes the degradation of the luminous efficacy of the plasma display panel.
In addition, since the liquid crystal filter is formed on the upper glass substrate of the plasma display panel, the thickness and the weight of the plasma display panel are increased.
Moreover, since the liquid crystal formed on the upper glass substrate of the plasma display panel is expensive, the fabrication cost of the plasma display panel is increased.